The present invention concerns improvements in measuring weft feeders for fluid jet looms (air or water looms), namely in those special weft yarn feeders for looms wherein the weft yarn, wound to form a reserve on a drum held stationary, is drawn by the loom with the help of a main nozzle and is measured, during its unwinding from said drum, by counting the turns drawn. Such weft feeders comprise an electromagnetic blocking unit, having a stem which is electromagnetically controlled to contact the edge of the drum, so as to stop the yarn from unwinding by engaging the same laterally.
More specifically, the present invention concerns an electromagnetic unit to block the weft yarn in said measuring weft feeders.
It is known that the performances demanded nowadays from an electromagnetic unit to block the weft yarn in measuring weft feeders are very high, and that the results obtained up to date with the already known units are by no means satisfactory, in that the conventional devices do not have characteristics answering the ever increasing requirements of modern weaving. In fact, a blocking unit of the type in question is required--in order to be really satisfactory--to perform over one thousand operations per minute, and to have a life corresponding to hundreds of millions of operations with no need for maintenance, an operating time below 5 ms with a stroke of at least 3-4 mm, and a high precision as to the exact moment at which the weft yarn is released. Furthermore, the magnet core of such unit has to be prevented from undergoing strong recoils (in practice, the recoils should not exceed 10% of its stroke) both in one direction and in the other.
In the event that the weft feeder should use a single electromagnetic unit to block the weft yarn--as is often the case, for reasons of simplicity and economy of construction--it has been found very appropriate to use said unit with the stem stopping the yarn from unwinding while the magnet is not energized. The use of a conventional electromagnetic unit, with a single coil attracting the core (and thus the stem) in order to block the weft yarn, involves in this case the need for the stem to return in the rest position by means of a spring, which latter should be sufficiently strong to reach the required operation speeds, but capable on the other hand to guarantee fairly contained recoils of the stem. It is needless to underline the difficulty of producing a spring with these characteristics: in fact, the use--in units of this type--of a spring for returning the stem in a weft yarn blocking position (rest condition of the electromagnetic unit) after the coil has been energized, involves on the one hand a high energizing time, since part of the force developed on the core is absorbed so as to actually win the resistance of the spring, and on the other hand it always determines, on reaching the blocking position after the coil has been de-energized, fairly strong recoils of the stem, which may cause even serious weft measuring errors, since the weft yarn will most likely pass under the stem just when this latter recoils.
It should be noted that these recoils, as the stem reaches its blocking position, are not easy to prevent, since the spring of the electromagnetic unit, as opposed to its coil, develops its minimum force on the stem just at the end of the stroke, when it is practically released and namely when the stem has to stop by striking against the inner stop. The use of even effective expedients, like that object of the Italian Utility Model application No. 22990 B/86 of the same Applicant, has limited this drawback, but has not allowed to reduce the recoils of the stem of the blocking unit to less than 10% its stroke, as it instead appears more and more indispensable for the most efficient working of measuring weft feeders in modern looms.